Referring to FIGS. 1 and 2, image sensor 10 includes a substrate 15 in which there is an array of pixels 20, and each pixel 30 is photosensitive. Referring now only to FIG. 2, depending on the architecture of the pixel 30, some regions of the pixel 30 may be intentionally shielded from light to improve optical isolation between adjacent pixels 30. In some cases, the light is inadvertently blocked as a result of the types of materials used to operate the pixel 30, such as control lines. For example, in interline CCD and CMOS image sensor pixels 30 dedicate a portion of the pixel area for reading out an image while another portion integrates the next image. This type of pixel requires the readout region to be shielded from light.
In all these cases, the pixel 30 will contain a light shield 40 that restricts where light is permitted to enter the pixel 30. However, this is undesired as it reduces the amount of light that can be sensed by the pixel 30 which reduces the overall sensitivity of the image sensor 10 and ultimately the sensitivity of camera in which it is included. The ratio of light sensitive area to pixel area is commonly referred to as the optical fill factor, and the area where the light shield 40 does not reside is referred to as the pixel aperture 50. Pixel apertures 50 are typically equally spaced to maintain uniform image sampling.
To overcome the loss in sensitivity due to the light shield 40, it has become common to incorporate a microlens 60 over each pixel 30, and the microlenses 60 are centered on the aperture 50. Light that may have otherwise been blocked or reflected by the light shield 40 is redirected by the microlens 60 through the aperture 50 and sensed by the pixel 30 thereby increasing sensitivity. Referring to FIG. 3, it is noted that there is a special case of full-frame or frame transfer type CCD image sensors 10 where the pixel 30 senses light through two different types optical materials, one of which is more transparent than the other. For example, a polysilicon material 70 will transmit less light than an indium tin oxide (ITO) material 80.
Referring to FIG. 4, in this case, a microlens 60 and a modified light shield 40 may be used to direct the light through the more transparent material, ITO 80, resulting in an overall gain in sensitivity. For color image sensors, redirecting all of the light through only one material type has the added benefit of reducing hue shifts across the array. This effect is most evident when the incident light angle varies across the array and is due to variations of each material's spectral (wavelength) response.
Referring to FIG. 5, although the presently known image sensor 10 is satisfactory, when incident light intersects the microlens 60 at an angle (θ), such as is common when introducing a camera lens, the focal point shifts away from the center of the aperture 50.
Referring to FIG. 6, if the incident light ray angle is sufficiently large, the focal point shifts far enough from the center of the aperture 50 to cause a fraction of the light to be blocked by the light shield 40 resulting in absorption or reflection losses. Thus, large and possibly abrupt reductions in sensitivity will be seen, primarily at the edges of the pixel array 20 (see FIG. 1) where the light angles are greatest. Referring to FIG. 7, the dependence of sensitivity with respect to incident light angle is commonly referred to as the array's ‘Angle Response.’
Referring to FIG. 8, to overcome this problem and extend the angle response, one practice is to gradually vary the position of the microlens 60 relative to the center of the aperture 50. In this practice, the microlens 60 is centered on the aperture 50 for pixels 30 located at the center of the array 20 (see FIG. 1), and pixel locations away from the center of the array 20 have microlenses 60 that are shifted on the aperture opening 50 toward the center of the array 20. The farther from center that a pixel 30 resides, the more the offset. The total offset is determined based on the characteristics of the camera lens used. Thus, this method works best in the case of a fixed lens system but the extent of the angle response improvement is still limited.
In the application of shift and tilt (view) cameras used in medium and large format photography, the incident light angles can be much larger (greater than 40 degrees) than can be accommodated by an offset microlens configuration. In addition, the light does not always intersect the sensor 10 in the conventional manner. For instance, the incident light angle at every pixel location could be in a single direction for one camera shot and just the opposite in the next. This environment requires the microlens to always be positioned centered on the light shield aperture in order to accommodate all possible camera shots.